Optical transmission systems including optical switching devices, control apparatuses, and methods

ABSTRACT

The optical systems of the present invention include optical switching device generally configured to control signal characteristic profiles over the pluralities of signal channels, or wavelengths, to provide desired signal characteristic profiles at the output ports of the device. Various signal characteristics that can be controlled include power level, cross-talk, optical signal to noise ratio, etc. The optical switching devices can include balanced demultiplexer/multiplexer combinations and switches that provide for uniform optical loss through the devices. In addition, low extinction ratio switches can be configured to provide higher extinction ratios.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/588,526, filed Jun. 6, 2000 now U.S. Pat. No. 6,771,905, which is acontinuation-in-part (“CIP”) of, and claims priority from, commonlyassigned U.S. Provisional Application Nos. 60/137,835 filed Jun. 7, 1999and 60/178,221 filed Jan. 26, 2000, all of which are incorporated hereinby reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

The present invention is directed generally to optical transmissionsystems. More particularly, the invention relates to optical switchingdevices, such as optical cross-connect switches, routers, add/dropmultiplexers, and equalizers for use in optical systems.

Digital technology has provided electronic access to vast amounts ofinformation. The increased access has driven demand for faster andhigher capacity electronic information processing equipment (computers)and transmission networks and systems to link the processing equipment.

In response to this demand, communications service providers have turnedto optical communication systems, which have the capability to providesubstantially larger information transmission capacities thantraditional electrical communication systems. Information can betransported through optical systems in audio, video, data, or othersignal formats analogous to electrical systems. Likewise, opticalsystems can be used in telephone, cable television, LAN, WAN, and MANsystems, as well as other communication systems.

Early optical transmission systems, known as space division multiplex(SDM) systems, transmitted one information signal using a singlewavelength in separate waveguides, i.e. fiber optic strand. Thetransmission capacity of optical systems was increased by time divisionmultiplexing (TDM) multiple low bit rate, information signals, such asvoice and video signals, into a higher bit rate signal that can betransported on a single optical wavelength. The low bit rate informationcarried by the TDM optical signal is separated from the higher bit ratesignal following transmission through the optical system.

The continued growth in traditional communications systems and theemergence of the Internet as a means for accessing data has furtheraccelerated the demand for higher capacity communications networks.Telecommunications service providers, in particular, have looked towavelength division multiplexing (WDM) to further increase the capacityof their existing systems.

In WDM transmission systems, pluralities of distinct TDM or SDMinformation signals are carried using electromagnetic waves havingdifferent wavelengths in the optical spectrum, i.e., far-UV tofar-infrared. The multiple information carrying wavelengths are combinedinto a multiple wavelength WDM optical signal that is transmitted in asingle waveguide. In this manner, WDM systems can increase thetransmission capacity of existing SDM/TDM systems by a factor equal tothe number of wavelengths used in the WDM system.

Optical WDM systems are presently deployed as in point-to-point WDMserial optical links (“PTP-WDM”) interconnected by electrical signalregeneration and switching equipment. At the electrical interconnectionsites in the PTP-WDM systems, all optical signals are converted toelectrical signals for processing. Electrical signals can be droppedand/or added at the site or can be regenerated and retransmitted onnominally the same or a different wavelength along the same fiber pathor switched to a different fiber path.

As would be expected, it can become extremely expensive to performoptical to electrical to optical conversions in PTP-WDM systems merelyto pass signals along to the transmission path. The cost of electricalregeneration/switching in WDM systems will only continue to grow withWDM systems having increasing numbers of channels and transmission pathsin the system. As such, there is a desire to eliminate unnecessary, andcostly, electrical regeneration and switching of information beingtransported in optical systems.

Current optical systems already benefit from the use of limitedcapability, optical switching devices. For example, optical add/dropmultiplexers provide optical access to a single transmission fiber toremove selected channels from a WDM signal. Optical add/dropmultiplexers eliminate the need to convert all of the optical signals inthe transmission fiber to electrical signals, when access to only aportion of the traffic being transmitted as optical signals is required.

As optical system capacity requirements continue to grow with demand, itwill become increasingly necessary for optical systems to evolve frompoint to point optical link toward multidimensional optical networks.Numerous optical switching devices have been proposed as alternatives toelectrical switching to enable multidimensional all optical networks.For example, U.S. Pat. Nos. 4,821,255, 5,446,809, 5,627,925 disclosevarious optical switch devices.

A difficulty with many proposed optical cross-connect switches is thatthe switches become overly complex with increasing numbers of opticalchannels and input/output ports. The interconnection of multiple fiberpaths, each carrying multiple channels, also becomes extremely difficultto manage effectively.

In addition, the interconnection of multiple fiber paths can introduceadditional complications in the system. For example, signal channelsbeing switched between different fiber paths can have different powerlevels that introduce channel to channel power variations in opticalpaths exiting the optical switching devices. The power variations inoptical switching devices are particularly troublesome, because thevariations can propagate through numerous optical paths and degrade theperformance throughout a portion of the system.

In addition, optical switching devices can also impose significantoptical losses on the signal channels passing through the devices. Asthe number of ports, i.e., size, and functionality of the opticalswitching devices increases, it generally becomes necessary to amplifythe signal channels to overcome splitting losses and other losses in theoptical switching device. However, optical amplification to overcomelosses associated with optical switching devices can further introducevariations in the signal channels passing through an optical amplifier.

Typically, optical amplifiers automatically control the characteristicsof signal channels passing through the optical amplifier using eitherAutomatic Power Control (APC) or Automatic Gain Control (AGC) schemes.In APC schemes, the amplifier gain will be varied according to the APCscheme to maintain the total output of the optical signal exiting theamplifier within a constant power range. As a result, if the number ofchannels varies, the individual signal channel powers can vary in APCschemes. In AGC schemes, the amplifier gain is maintained within aconstant gain range. Therefore, if the signal channel powers or thenumber of signal channels varies, then the total output power of theoptical signal channel will vary as the overall gain of the amplifier ismaintained within its constant gain range. As with APC schemes, theindividual signal channel powers can vary in AGC schemes.

Traditional AGC and APC schemes are typically not effective in opticalswitching device configurations. The ineffectiveness is because signalchannels are often combined from multiple input optical paths, which canhave different signal powers. Thus, while the signal channels in anygiven input optical path may be controlled, variations in signal powerbetween the input paths can cause signal channel power variations in oneor more of the output optical paths.

Signal channel power variations also can be inherently produced as aresult of the optical switching device design. The various processesperformed in the devices, such as demultiplexing, splitting, switching,adding, dropping, coupling, and multiplexing of various signal channelscan each introduce variations in the signal channel power levels.

For example, in many optical systems and component designs it is commonto include symmetrically designed demultiplexers and multiplexers. Thesymmetrical demux/mux construction provides for streamlinedmanufacturing of the products and the potential for bi-directional usein bi-directional systems. However, symmetrical demux/mux configurationscan produce signal channel power variations, if deployed with alloptical switching devices.

Analogously, dissimilar demux/mux configurations are often deployed invarious optical switching devices. For example, wavelength selectivedemultiplexers can be used with non-wavelength selective combiners, suchas N:1 couplers. These configurations can also introduce signal channelpower variations.

Depending upon the number of channels in the WDM system, one or morestages of non-wavelength selective splitting and combining can be usedalong with various filtering techniques applied to each wavelength. Forexample, see U.S. Pat. No. 5,446,809 (the “'809 patent”), which isincorporated herein by reference. Unfortunately, as the number ofchannels in WDM systems continues to increase, non-wavelength selectivesplitting and coupling can become impractical even when opticalamplification is used to overcome the passive losses.

Another source of signal channel degradation in optical switchingdevices occurs when the switches incompletely block signal channels,thereby allowing leakage of unwanted signal channels through the device.Leakage of unwanted signal channels will degrade the signal quality ofthe signal channels being passed through the devices by creatingcross-talk interference. Significant levels of cross-talk can destroythe information carried by signal channels being passed through thedevice. The amount of crosstalk that occurs through an on/off switch canbe characterized by the extinction ratio of the switch, which is theratio of power transmitted through the switch in the on state over theoff state.

There are numerous types of optical switches, or gates, such asmechanical, thermo-, acousto-, and electro-optic, doped fiber andsemiconductor gates, and tunable filters, that are available for use inoptical switching devices. Thermo-, acousto-, and electro-optic switchesare appealing, because of the relatively low cost and solid statecharacteristics. However, these switches often have extinction ratiosonly on order of ˜20 dB. The low extinction ratios can introduceunacceptable levels of cross-talk in optical systems. As such, thesetypes of switches are typically limited to use in systems in which minorsignal degradation can be tolerated or the degradation does notaccumulate from multiple switches.

Conversely, mechanical line or mirror switches can have excellentextinction ratios, but the moving parts associated with mechanicallymoving and aligning the switch can pose a reliability problem over thelong term. Doped fiber and semiconductor on/off gates can provide goodextinction, but require active components to maintain the gateconfigurations, which increases cost and decreases reliability. Tunablefilters can also provide good extinction ratio in a switch mode, but thefilters may require active components that have maintainable, stabletuning characteristics.

The various signal degradation mechanisms that can be introduced in anoptical system by prior art optical switching devices have been acontributing factor to the industry's inability to develop and deployreliable optical switching devices to enable the creation ofall-optical, or “transparent”, networks. Accordingly, there is a needfor optical systems including optical amplifiers and optical switchingdevices that provide increased control over signal channels beingswitched, or routed, through the system.

As the need for high capacity WDM systems continues to grow, it willbecome increasingly necessary to provide all optical networks thateliminate the need for and expensive of electrical regeneration toperform signal routing and grooming in the networks. The development ofmulti-dimensional, all-optical networks will provide the cost andperformance characteristics required to further the development of highcapacity, more versatile, longer distance communication systems.

BRIEF SUMMARY OF THE INVENTION

The apparatuses and methods of the present invention address the aboveneed for higher performance optical systems. Optical systems of thepresent invention generally include at least one optical processingdevice, such as optical cross-connect switches and routers, as well asadd/drop multiplexers, disposed along an optical path betweentransmitting and receiving optical processing nodes.

Optical signals from the transmitting nodes pass through the opticalswitching devices via input and output ports to the receiving nodes. Invarious embodiments, the optical switching device includes opticalamplifiers, such as EDFAs, configured to provide automatic gain & powercontrol (AGPC) over the output power of signal channels passing throughthe optical amplifier. AGPC allows signal channels from diverse opticalpaths to be combined without incurring a substantial performance penaltydue to non-uniform signal power levels in the combined signal channels.

The optical switching devices generally are configured to control thesignal characteristic profile over the pluralities of signal channels,or wavelengths, and provide a desired signal characteristic profile atthe output ports of the switching devices. Various signalcharacteristics controlled in the devices include power level,cross-talk, optical signal to noise ratio, etc. The optical switchingdevices can include balanced demultiplexer/multiplexer combinations andswitches that provide for uniform optical loss and provide for variablepower control of the signal channels passing through the devices. Inaddition, low extinction ratio switches can be configured to providehigher extinction ratio switching devices.

Accordingly, the present invention addresses the aforementioned needs byproviding optical systems, apparatuses, and methods with increasedcontrol over signal channels being combined from different opticalpaths. These improvements provide for the use of optical switchingdevices in optical systems without incurring a substantial decrease inthe performance of the system. These advantages and others will becomeapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings forthe purpose of illustrating embodiments only and not for purposes oflimiting the same, wherein:

FIGS. 1–2 c illustrate exemplary optical systems including exemplaryoptical switching devices;

FIG. 3 illustrates an exemplary optical switching device;

FIGS. 4 a–4 b illustrate exemplary optical amplifier configurations;

FIGS. 5 a–6 d illustrate exemplary optical switching devices;

FIGS. 7 a–9 b illustrate exemplary demultiplexer and multiplexerconfigurations;

FIGS. 10 a–10 b illustrate exemplary optical switching devices;

FIGS. 11 a–11 b illustrate exemplary multiple filter packages; and,

FIGS. 12–14 illustrate exemplary optical switch elements.

DESCRIPTION OF THE INVENTION

Optical systems 10 of present invention include at least one opticalswitching device 12 interconnecting two or more other optical processingnodes 14. Various guided or unguided transmission media, such as opticalfiber 15, can provide an optical link 16 allowing for opticalcommunication between the nodes 12. The optical switching devices 12generally include optical routers, optical cross-connect and lineswitches, add/drop devices, and other devices interconnecting opticalfibers that provide the optical links 16. In the context of the presentinvention, an optical switching device 12 receives an optical signal atan input port and can pass or substantially block the passage of one ormore optical signal wavelengths, or signal channels, λ_(i) included inthe optical signal to an output port, without converting the opticalsignal to electrical form.

The system 10 can be configured in various linear and multi-dimensionalarchitectures (FIG. 1) and controlled by a network management system 18.One or more optical amplifiers 20 can be disposed periodically along theoptical links 16, e.g., every 20–120 km, to amplify attenuated opticalsignals during transmission between nodes 14 and/or proximate to otheroptical components to provide gain to overcome component losses. One ormore optical fibers can be disposed to provide multiple optical link 16between nodes 14 along a common optical path. In addition, each fibercan carry uni- or bi-directionally propagating optical signals dependingupon the system 10 configuration.

Generally, the optical processing nodes 14 can include one or moreoptical transmitters 22 and optical receivers 24, as well as opticalswitching devices 12. Various combinations of optical switching devices12, transmitters 22, and receivers 24 can be included in each processingnode 14 depending upon the desired functionality in the nodes 14.

The optical transmitters 22 and optical receivers 24 are configuredrespectively to transmit and receive optical signals including one ormore information carrying optical signal wavelengths, or signalchannels, λ_(i). In the present description, the term “information”should be broadly construed to include any type of information that canbe optically transmitted including voice, video, data, instructions,etc.

The transmitters 22 used in the system 10 generally will include anarrow bandwidth laser optical source, such as a DFB laser, thatprovides an optical carrier. The transmitters 22 also can include othercoherent narrow or broad band sources, such as sliced spectrum or fiberlaser sources, as well as suitable incoherent optical sources asappropriate. Information can be imparted to the optical carrier eitherby directly modulating the optical source or by externally modulatingthe optical carrier emitted by the source. Alternatively, theinformation can be imparted to an electrical carrier that can beupconverted onto an optical wavelength to produce the optical signal.The information can be amplitude, frequency, and/or phase modulatedusing various formats, such as return to zero (“RZ”) or non-return tozero (“NRZ”) and encoding techniques, such as forward error correction(“FEC”).

The optical receiver 24 used in the present invention will be configuredto correspond to the particular modulation format used in thetransmitters 22. The receiver 24 can receive the signals using variousdetection techniques, such as coherent detection, optical filtering anddirect detection, and combinations thereof. Employing tunabletransmitters 22 and receivers 24 in the optical nodes 14 in a network,such as in FIGS. 1–3, can provide additional versatility in configuringthe network architecture of the system 10.

The transmitters 22 and receivers 24 can be also connected tointerfacial devices 25, such as electrical and optical cross-connectswitches, IP routers, etc., to provide interface flexibility within, andat the periphery of, the optical system 10. The interfacial devices 25can be configured to receive, convert, and provide information in one ormore various protocols, encoding schemes, and bit rates to thetransmitters 22, and perform the converse function for the receivers 24.The interfacial devices 25 also can be used to provide protectionswitching in various nodes 14 depending upon the configuration.

Optical combiners 26 can be used to combine the multiple signal channelsinto WDM optical signals. Likewise, optical distributors 28 can beprovided to distribute the optical signal to the receivers 24 _(j). Theoptical combiners 26 and distributors 28 can include various multi-portdevices, such as wavelength selective and non-selective (“passive”),fiber and free space devices, as well as polarization sensitive devices.For example, circulators, passive, WDM, and polarizationcouplers/splitters, dichroic devices, prisms, diffraction gratings,arrayed waveguides, etc. can be used alone or in various combinationsalong with various tunable or fixed wavelength transmissive orreflective, narrow or broad band filters, such as Bragg gratings,Mach-Zehnder, Fabry-Perot and dichroic filters, etc. in the opticalcombiners 26 and distributors 28. Furthermore, the combiners 26 anddistributors 28 can include one or more stages incorporating variousmulti-port device and filter combinations to multiplex, demultiplex,and/or broadcast signal channels λ_(i) in the optical systems 10.

As shown FIG. 2 a, the optical switching device 12 can be embodied as anoptical router, or cross-connect switch, including a plurality of inputports 30 _(I) and output ports 30 _(O). In FIG. 2 a, the device 12,transmitters 22 and receivers 24 are shown as separate nodes forconvenience. It will be appreciated that devices 12, the transmitters 22and receivers 24 can be located in different or the same physical node14 depending upon the system configuration. The device 12 is shown witheach of the input ports 30 _(I) and output ports 30 _(O) opticallyconnected via an optical switch path 32 and a switch element 34 disposedalong one of the optical switch path 32. It will be appreciated thatdevice 12 can include multiple optical switch paths between input andoutput ports and that some input and output ports may not be opticallyconnected.

The switch element 34 will generally be disposed along each opticalswitch path 32 connecting the input and output ports to allow selectivepassage or blockage of the optical signals in the optical switch paths32. The switch element 34 can include wavelength selective ornon-selective on/off gate switch elements, as well as variable opticalattenuators configured to have a suitable extinction ratio as will befurther discussed. The switch elements 34 can include signal path, aswell as multiple path elements employing polarization, interferometry,or other effects to perform the gating and/or variable attenuationfunction. Wavelength selective switch elements 34 _(s) can be used withvarious types of distributors 28 and combiners 26. Whereas,non-selective switch elements 34 _(ns) will generally require some levelof demultiplexing in the distributors 28, unless only optical lineswitching is performed.

The optical switching device 12 can be configured to selectively switchand route signal groups that may include individual signal channelsand/or groups of signal channels including up to all of the signalchannels in an optical signal λ_(I,1-N) or line switch the opticalsignal provided to the input port 30 _(I) to one or more of the outputports 30 _(O). Optical switching devices 12, such as cross-connects,routers, and add/drop multiplexers, that can be used to switch and routegroups of signal wavelengths, or signal channels, are described U.S.patent application Ser. No. 09/119,562, which is incorporated herein byreference.

The devices 12 can be configured to perform various other functions,such as signal group filtering, power equalizing, adding telemetry andmarkers to the signal channels, etc., in the system 10. In variousembodiments, the switch elements 34 can include binary on/off gates incombination with variable attenuators or variably attenuating on/offgates. Variable attenuation in combination with optical amplificationand on/off switching in the switch elements 34 provide for channelfiltering and power equalization, or gain trimming, of the signal grouppower passing through the device 12. In various embodiments, variableoptical attenuators, such as those commercially available fromJDS-Uniphase, Inc., can be used to variably control the signal groupoutput power from the switch element 34 to provide for gain equalizationwith binary on/off gates. Alternatively, optical amplifiers 20 can beconfigured to operate as variably attenuating on/off gates, as will befurther described.

As shown in FIG. 2 b, the devices 12 can be deployed in 1×1, in additionto N×M, configurations along the optical link 16 to provide signalgroup/channel filtering and equalization. In 1×1 configurations, thedistributor 28 can demultiplex optical signals into respective signalgroups that can be separately filtered or equalized by the switchelements 34 before being recombined into an output signal, as will befurther described.

In addition, distributors 28 and combiners 26, typically passivesplitters and couplers, respectively, can be provided on the input andoutput to provide drop access for receiving the channels from the fiber16 and/or to add channels to the fiber 16. The switch elements 34 in thedevice 12 can be configured to substantially block or pass thosechannels that were received from the drop access depending upon whetherwavelengths are being reused or multicast. Similarly, the switchelements 34 can be configured to block wavelengths including the samesignal channels as those being added to the system 10 after the device12. It will be appreciated that wavelength selective add and/or dropmultiplexers also can be used along with the device 12 or the device 12can be configured to provide dedicated add/drop channels. However, somewavelength selective add/drop embodiments can limit the flexibility inreconfiguring the system 10.

Alternatively, the device can be configured in 2×1, 2×2 (FIG. 2 c), and1×2 configurations to accommodate adding and/or dropping of signalchannels using the device 12. Optical transmitters 22 and/or receivers24 can be respectively coupled to the input port 30 _(I) and output port30 _(O) to selectively add and/or drop signal channels using the device12. The distributors 28 can include various combinations of passivesplitters, filters, and demultiplexers to provide the input signals tothe switch elements 34, as will be further described.

In the various add/drop embodiments of the devices 12, the dropreceivers 24 and add transmitters 22 can be locally or remotely locatedfrom the switching device 12. In various embodiments, add and drop portsof the device 12 can be directly connected with a local system, such asa ring, as shown in FIG. 1. In addition, tunable transmitters 22 andreceivers 24 are particularly advantageous in add/drop configurations ofthe device 12. In these embodiments, the device 12 can be dynamicallyreconfigured along with the transmitter and receiver wavelengths toreconfigure the signal channel plan of the system 10.

Optical switching devices 12, particularly those having output ports 30_(O) provided with signal channels from multiple input ports 30 _(I),can significantly impact the performance of the system 10. For example,variations in signal channel power in the combined optical signals canaffect the performance of optical amplifiers 20 and receivers 24disposed along the optical link 16 connected to the output ports 30_(O). Therefore, it is important to control the signal channel powerlevels λ_(i) not only within each optical signal provided to device 12,but across all optical signals that pass through the device 12 to outputfibers 30 _(O).

The devices 12 may include one or more amplifiers 20 in variousconfigurations relative to the input and output port 30 to overcomeoptical losses associated with the optical switching devices 12, inaddition to along the optical link 16. For example, it may be necessaryto amplify the signal channels to overcome splitting losses in thedevices 12 having multiple output ports 30 _(I) to which the signalchannels may be multicasted.

As shown in FIG. 3, the optical amplifiers 20 can be located before orafter the distributor 28 or switch elements 34. The optical amplifiers20 can also be incorporated as the switch elements 34 to provideconsolidated amplification, on/off switching, and variable attenuationof signals passing the device 12. For example, in the “off”state/position, an unpumped erbium doped fiber amplifier will absorboptical energy, thereby blocking signal channels λ_(i) from passingthrough the switch element 34 to the output port 30 _(O). Whereas, inthe “on” state/position, an optically pumped EDFA will amplify thesignal channels λ_(i), thereby serving as an amplifier and on/off gatepassing the signal channels λ_(i) to the output port. Furthermore, thepump supplied to the optical amplifier can be variably controlled to notonly turn the gate on, but to control the output power of the signalgroup from the switch element. The consolidated functionality of theoptical amplifiers 20 as switch elements 34 provides for signal channelpower equalization and channel filtering in 1×1 and N×M configurationsof the device 12. In addition, telemetry and device identifierinformation, etc. can be received and/or imparted to the signal channelspassing through the device 12 by imparting information onto the pumppower being supplied to the amplifier 20.

Optical amplifiers 20 in the device 12 can be disposed between the inputand output ports and configured to provide automatic gain & powercontrol (AGPC) over the output power of signal channels λ_(i) passingthrough the optical amplifier 20. By controlling the signal channeloutput power and the loss in the device 12, the performance of theoverall system 10 can be maintained, when signal channels from multiplepaths are combined using the device 12.

In various embodiments, the gain of the amplifier 20 is controlled baseda gain set point G_(s), which itself can be fixed or controlled based onthe number of signal channels in the optical signal and the input powerof the signal channels. As generally shown in FIG. 4 a, optical signaldistributors 28, such as optical taps, can provided before and/or afteran amplifying medium 36 to monitor the input and output signal channelpowers. The distributors 28 are optically connected to respectiveoptical to electrical converters 38 that can be configured to provideelectrical signals corresponding to the signal channel input and outputpowers and the number of signal channels.

The amplifier gain can be calculated from the input and output powerusing a divider circuit 40, which can then be compared to the gain setpoint G_(s) for the optical amplifier 20 using a comparator circuit 42.The output of the comparator circuit 42 is used to control the powersupplied to the optical amplifier 20 by a power source 44, typically bycontrolling the drive current to the power source 44. A centralprocessor 46 can be used to reset the gain set point G_(s) based on theamplifier input power and the number of signal channels passing throughthe amplifier 20.

Alternatively, the amplifier gain performance can be calibrated as afunction of the power supplied by the power source 44 and the inputpower and the number of signal channels. The overall input power wouldbe detected and compared in the comparator 42 to an input power setpoint P_(s). The output of the comparator circuit 42 would again be usedto control power provided by the power source 44. The central processor46 calculates the number of signal channels passing through theamplifier based on an expected range of per channel power levels andresets the input power set point P_(s) to accommodate variations in thenumber of signal channels. Similar configurations can be provided basedon monitoring the signal power at the amplifier output. While theamplifier control loops generally have been described in an analogfashion; digital implementations of the control loop can also beemployed.

FIG. 4 b illustrates a more specific exemplary embodiment of the opticalamplifier 20 including an amplifying fiber 50, such as an erbium dopedfiber amplifier, supplied with optical energy from pump sources 52. Thepump sources 52 can provide optical energy, “pump power” in one or morepump wavelength λ_(pi), which are counter-propagated and/orco-propagated with signal channels λ_(i). For example, pump power in the980 nm and/or 1480 nm wavelength ranges can be provided to the erbiumdoped fiber 50 to amplify signal channels in the 1550 nm range.

Passive couplers 54, such as high ratio optical taps, can be used tosplit a percentage of the optical signal to photodiodes 56 provide anelectrical signal corresponding to the optical signal power to thecentral processor 46. Signal channel counting devices 58 can be used atthe input and/or output of the amplifier 20 to determine the number ofsignal channels passing through the amplifier 20. The central processor46 controls the gain and power set points, G_(s) and P_(s), based on thenumber of channels and the optical signal power. The central processors46 also can compare the channel information from the signal countingdevices 58 with channel information from the network management system18 to verify system performance and take appropriate actions, ifdiscrepancies occur.

Signal channel counting devices 58 can be embodied as one or morephotodiodes 56 with fixed or tunable filters or as optical spectrumanalyzers configured to count the number of signal channels. Thecounting devices 58 also can be configured to provide the individualsignal channel power levels. Alternatively, the number of signalchannels also can be transmitted along with the signal channels. Inthese embodiments, the counting devices 58 also can include an opticalor electrical filter to separate the signal channel number identifiersignal. For example, the number of signal channels could be transmittedon a low frequency tone to allow it to be detected separately from thesignal channels and with lower cost electronics. A single tone could beused to indicate the number of channels, or each channel could have itsown channel identifier tone. In addition, the number of signal channelscan be transmitted on a separate wavelength or in the overheadinformation carried by the signal channels. Channel identificationinformation also can be included along with the system supervisoryinformation that is transmitted to each network element, i.e.,amplifiers 20, nodes 14, etc. and/or calculated from the total power.

In accordance with present invention, if the number of signal channelspassing through the amplifier 20 increases or decreases, the AGPC loopwill initially act to maintain the gain of the amplifier 20 at the gainset point G_(s). Contemporaneously, the central processor 46 will detecta change in the number of signal channels and the input power and adjustthe gain set point G_(s) to maintain the desired output power for thesignal channels exiting the amplifier 20. Additionally, if the signalchannel input powers vary and the number of signal channels remainsconstant, the central processor 46 will adjust the gain set point tomaintain the desired signal channel output power. Thus, signal channelsfrom diverse optical paths can be combined with minimal performancepenalty due to non-uniformly amplified signal channels.

Furthermore, when channel identifier tones are used, the amplifiercontrol scheme can be synchronized to minimize unnecessary changes tothe amplifier set points. For example, the number of channels passingthrough the amplifier can be verified as described before the gain orpower is varied in response to detected power variations.

Non-uniformities also can be introduced across the signal channel λ_(i)range, because of varying optical losses that can occur in thedistributors 28, combiners 26, and switch elements 34 used in thedevices 12. The variations are often introduced because the wavelengthselective devices used to separate WDM signals generally exhibitdifferent performance in terms of extinction ratios, loss, and otherproperties between transmission and reflection. As such, prior devicescan introduce signal channel power variations that can degrade systemperformance.

In the present invention, optical switching devices 12 includesdemultiplexers 60, multiplexer 62, and passive splitter 64 and couplers66 that are configured in various combinations along with the switchelements 34 to balance the optical loss over the signal channel λ_(i)range. The distributors 28 and combiners 26 can be deployed as one ormore stages in various configurations depending upon the desired device12 characteristics including whether wavelength selective switchelements 34 _(s) or non-selective (“non-selective”) switch elements 34_(ns) are used. FIGS. 5 a–6 d depict exemplary embodiments of thedevices 12.

In FIG. 5 a embodiments, passive splitters 64 are provided at the inputports 30 _(Ii) of the device 12 to passively split the input signalsλ_(Ii,1-Ni) and provide the entire input signal to each output port, ora subset thereof, via the associated selective switch element 34 _(s)and combiners 26. Passive couplers 66 can be provided at the outputports 30 _(oi) to combine the signals passing through selective switchelements 34 _(s). Output signals λ_(oi,1-mi) are provided at each outputport 30 _(oi) for further transmission along the optical path orprovided to receiver 24 collocated with the device 12 at combinedswitching and destination nodes 14.

Wavelength selective filters, such as in the '809 patent, can be used toselectively pass or substantially prevent the passage of the individualwavelengths in the input signal. Alternatively, a demultiplexer 60 andmultiplexer 62 in combination with non-selective switch elements 34_(ns) can be used to selectively pass or substantially prevent thepassage of signal groups as shown in FIG. 5 b.

Conversely, in FIG. 6 a embodiments of the device 12, demultiplexers 60can be provided to demultiplex the input signals received from the inputports 30 _(Ii) into a plurality of signal groups. The signal groups canbe split using passive splitters 64 and provided via switch elements 34and couplers 66 to respective output port 30 _(o). The switch elements34 in these embodiments can be non-selective switch elements 34 _(ns) orwavelength selective switch elements 34 _(s). Additional demultiplexingcan be performed after the splitter 64 to further separate the signalgroups, if so desired.

Various other combinations of splitters 64 and demultiplexers 60 andcouplers 66 and multiplexers 62 are possible in the present invention.For example, in FIG. 6 b embodiments, one or more splitter stages 64 canbe used to provide replicate signals corresponding to the number ofsignal groups. The replicate signals can be split further and providedto selected, or all, output ports of the device 12. In FIG. 6 bembodiments, additional amplification may be necessary to overcome themultiple splitting losses in the device 12. In these embodiments, thesignal channels can be double passed through one optical amplifier 20using optical circulators 68 to eliminate the need for separateamplifiers before and after the switch elements, as shown in FIG. 6 c.Other multi-port device configurations can be used in lieu of, orcombination with the circulator 68 in FIG. 6 c embodiments. Also, asignal group filter 70 can be included in the double configuration toincrease the rejection of unwanted signal groups.

The device 12 can be configured to provide one input port to one outputport, multicast, and broadcast switching capabilities. For example, 1×Nline switches 65 can be used in lieu of, or in combination with,splitter 64 and switch element 34 embodiments, as shown in FIG. 6 d. Theline switch 65 is used to selectively direct the entire demultiplexedsignal group to only one of the output ports of the device 12. Inaddition, it is not necessary that each input port 30 _(i) be connectedto each output port 30 _(o) or that only one type of switch element 34be used in the device 12. In some instance, it may be desirable toprovide connectivity between only a subset of the input 30 _(i) andoutput ports 30 _(o) or to provide additional ports for future systemupgrades.

The distributors 28, switch elements 34, and combiners 26 are selectedto provide a substantially uniform optical performance over the signalchannel range. When non-selective switch elements 34 _(ns) are used inthe device 12, the optical signals provided to the input port 30 _(I)can be demultiplexed into signal groups of a desired optical switchinggranularity before switching. The non-selective switch element 34 _(ns)will generally provide a substantially uniform loss over the signalchannel range because it either passes or blocks the entire opticalsignal. Therefore, it is generally necessary to balance the optical lossof the demultiplexer and multiplexer (“demux/mux”) over the signalchannel range.

In various embodiments, the optical amplifiers 20, associated with thedevices 12, as well as variable optical attenuators, can be configuredto compensate for non-uniform loss within the device 12. In this manner,the demultiplexer, multiplexer, and switch elements 34 can be optimizedfor other signal characteristics and the optical amplifiers 20 willbalance the overall loss through the device 12. The optical amplifiers20 can be variously located proximate the demux/mux and switch elements,as described with respect to FIG. 3.

The demux/mux loss can generally be balanced by exposing each wavelengthto the same number of band-pass (transmission) filters, as well as thesame number of band-stop (reflection) filters; and, thus, the sameplural total number of filter stages. The band-pass and band-stopfilters can include appropriately configured high-pass or low-passfilters. In addition, it is not necessary that number of pass-bandstages be equal to the number of stop-band stages.

FIGS. 7–10 show exemplary demultiplexers and multiplexers that providefor a substantially uniform optical loss across the signal channelrange. While FIGS. 7–10 show Bragg gratings as exemplary filters, otherfilters including those described herein can be substituted asappropriate. Also, the various signal group filters can be the same ordifferent design and can used to filter signal groups that includedifferent number of signal channels.

FIGS. 7 a and 7 b show linear demultiplexers 60 and multiplexers 62,respectively, for use in the present invention. As shown in FIGS. 7 aand 7 b, each signal groups 1-N is selectively filtered using a signalgroup filters 70, such as a Bragg grating 72 and removed via circulator68. The signal group filters 70 used in the linear configurations areshown as Bragg gratings 72; however, Fabry-Perot filters also can used,as well as WDM couplers, etc., as previously described.

Similarly, FIGS. 8 a and 9 a and 8 b and 9 b show paralleldemultiplexers 60 and multiplexers 62, respectively, for differingnumbers of signal channels or groups of signal channels. The separationof the signal groups in these configurations is performed in parallel,as opposed to serially. While parallel designs are generally moreefficient than linear designs, multiple signal group processing inparallel stages can introduce signal cross-talk.

FIGS. 10 a and 10 b show single stage demultiplexers 60 and multiplexers62 in combination with wavelength selective switching elements 34.Wavelength controllers 73 are provided to communicate with the centralprocessor 46 and control the wavelength of the filters 70/72 inaccordance with switch configurations provided by the network managementsystem 18.

In FIGS. 10 a and 10 b configurations, both the demux/mux stages arebalanced, as well as the switch element 34 stages. Conversely, invarious embodiments, wavelength selective switch element stages and thedemux/mux stages provide overall balance of the optical switching device12; however, neither the switch element 34 stages nor the demux/muxstages are separately balanced.

It will be appreciated that wavelengths can be selectively switched invarious orders depending upon the system configurations. For example,alternating the wavelengths or wavelength groups switched in each stagescan increase the rejection of adjacent wavelengths. Likewise, switchingmultiple wavelengths or wavelength groups in each stage can be used tominimize the overall loss through the switching elements. In addition,the switching and/or demux configurations can be varied depending uponthe wavelengths used in the optical system and the type of wavelengthselective devices used. For example, serially connected Bragg gratingscan be arranged to reduce cladding mode reflections of shorterwavelengths by longer wavelength gratings that can result in crosstalkinterference as will be further discussed.

Bragg grating filters 72 can be reliably manufactured to provide bothvery narrow (<0.1 nm) and extremely broad wavelength filters (>20 nm)for use in the present invention. Also, the reflective wavelength ofBragg grating filters 72, as well as other filters, often has atemperature dependence that allows the filters 70 to be thermally tuned.However, the temperature dependence also can cause the filterwavelengths to shift undesirably during operation. As such, it isgenerally necessary to control the temperature of the grating or packagethe grating such that the thermally variations are offset. Bragg gratingpackaging generally relies on the additional dependence of Bragggratings on strain to offset the temperature variations, such asdescribed in U.S. Pat. No. 5,042,898.

In demultiplexers 62 and switch elements 34, it may be necessary tocontrol the temperature or tune numerous signal group filters 70 withinthe device 12. In various embodiments, wavelength controllers 73 providetemperature control over multiple signal group filters 70, such as Bragggratings 72, using a common cooling element 74, such as athermo-electric cooler, and individually heating elements 76. Eachfilter 70 is surrounded by high thermal conductivity material 78, suchas metallic compounds, to ensure good thermal contact with the heatingand cooling elements.

Conversely, adjacent filters 70 are separated by low thermalconductivity material 80, such as polymers, etc. to thermally isolateeach filter 70, thereby allowing individual temperature control via theheating elements 76. One or more temperature sensors 82, such asthermocouples and thermistors, can be provided adjacent to the filters70 to monitor the temperature and provide for feedback control of theheating elements 76 and/or cooling elements 74. FIGS. 11 a and 11 billustrate cross-sectional, side views of two parallel filter packages.FIG. 11 a embodiments provide for redundant cooling elements on oppositesides of the filter 70. Whereas, in FIG. 11 b embodiment, onedirectional heat transfer via the high thermal conductivity material 78is provided to the cooling element 74, which also can be deployed in aredundant configuration.

Various non-selective switch elements 34 _(ns) can be used in presentinvention, such as mechanical line and micro-mirror (“MEM”) switches,liquid crystal, magneto-optic, thermo-optic, acousto-optic,electro-optic, semiconductor amplifier, etc., as well as erbium dopedfiber amplifier switch elements previously described. Alternatively, theswitch elements 34 can employ wavelength selective multi-port devicesand filters, such as WDM couplers, circulators, Bragg gratings,Mach-Zehnder, Fabry-Perot and dichroic filters, as shown in theexemplary embodiments of FIG. 10. As previously described, the switchelement 34 can be configured strictly as on/off gates and/or variableattenuating on/off gates. For example, the nonselective switch 65 shownin FIG. 6 b can be embodied as any of the foregoing switch types, andcan include an optical attenuator to control the signal power.

It is generally desirable to use a switch element 34 that has a highextinction ratio, e.g., >40 dB, to substantially prevent the passage ofa signal through the switch in the off position. Signal leakage throughswitches in the off position can severely degrade signals passingthrough the device 12 by causing crosstalk interference between thesignals. Lower extinction ratio, <40 dB, switch elements 34 thatsignificantly attenuate can be deployed in various configurationsdepending upon the system configuration.

The extinction ratio required to prevent significant system performancedegradation from cross-talk interference depends, in part, on the systemconfiguration. For example, lower extinction ratio switch elements 34may be sufficiently effective to prevent significant crosstalkdegradation of the signals, when there are no other sources ofsignificant crosstalk in the system. Conversely, when multiple devices12 or other potential sources of crosstalk, such as mux/demuxconfigurations, are concatenated, the extinction ratio of the devices 12generally has to be higher to prevent an unacceptable cumulativecrosstalk degradation of the system performance.

In the present invention, low extinction ratio switch elements, such asa thermo-optic and electro-optic switches, as well as wavelengthselective switches with low extinction ratios, e.g., <99% reflective,can be configured to provide higher extinction ratio devices. As shownin FIG. 12, the switch element 34 includes a low extinction ratio on/offswitch element 34 ₁, such as a variable optical attenuator, that isserially connected to a saturable absorber 84. The saturable absorber 84is designed to absorb low levels of optical energy characteristic ofoptical signal leakage, when non-selective switch element 34 ₁ is in theoff configuration. However, the saturable absorber 84 is furtherdesigned to saturate at higher levels of optical energy, therebyallowing the remaining optical energy in the optical signal to passthrough the switch element 34. Suitable saturable absorbers for use inthe present invention include erbium doped fiber, semiconductoramplifiers, or other saturable media.

Lower extinction ratio switch elements 34 also can be embodied as 2×2crossbar switch having an input port (In) and an output port (Out)alternately connected to first and second input/output ports, In/Out₁and In/Out₂, respectively. The first and second input/output ports areoptically connected via an optical fiber path 16. An optical isolator 86is provided in the path 16 to prevent optical energy from passing fromthe second input/output port to the first input/output port. In a first(“on”) switch configuration shown in FIG. 13 a, the optical signalpasses from the input port through the first crossbar 88 ₁, the firstand second input/output port, the second crossbar 88 ₂, and exits theswitch element 34 via the output port. In a second (“off”)configuration, shown in FIG. 13 b, the optical signal passes from theinput port through the first crossbar 88 ₁ and the second-input/outputport and is prevented from reaching the first input/output port by theisolator 86. Any signal leakage from the input port to the firstinput/output port will be further attenuated by the leakage path, shownas dashed lines, from the second input/output port to the output port ofthe switch 34.

Various switching elements, such as liquid crystal switches, can havepolarization dependent extinction ratios that can introduce varyingamounts of crosstalk into the system 10. FIG. 14 illustrates embodimentsof the present invention to decrease the polarization dependence ofthose switch elements 34. Signals passing through the switch element 34are reflected by a Faraday rotator 90, which also rotates thepolarization of the signals. The polarization rotated signal passthrough the switch element 34 a second time, so that each portion of thesignal has passed through the switch element in both polarizations. Inthese embodiments, not only can the polarization dependence of theoverall be eliminated, but the device has an effective extinction ratiothat is higher than a single pass through the switch element 34.

In the operation of the present invention, input signals including oneor more signal channels are provided to the device 12 at various inputports from various transmitters in the system 10. The device 12 isconfigured to selectively switch the signal channels in signal groupsincluding one or more signal channels to output ports leading toreceivers at the signal destination. The device 12 balances the opticalsignal characteristics through the device, such that signal channelscombined from various input ports have substantially equal signalcharacteristics including signal power, cross-talk noise, etc.

Those of ordinary skill in the art will appreciate that numerousmodifications and variations can be made to specific aspects of thepresent invention without departing from the scope of the presentinvention. It is intended that the foregoing specification and thefollowing claims cover such modifications and variations.

1. An optical transmission system comprising: at least one opticaltransmitter configured to transmit at least one optical signal channelvia an optical signal along an optical path; at least one opticalreceiver configured to receive at least one optical signal channel fromsaid optical path; and, an optical switching device disposed along saidoptical path including: a demultiplexer configured to demultiplex theoptical signal provided to an input port into a plurality of signalgroups, each signal group including at least one signal channel, aswitch element configurable to pass or prevent the passage of at leastone of the signal groups and variably control the optical power of thesignal groups passing though said switch element, and a multiplexerconfigured to multiplex the signal groups passed by said switch elementinto an output signal through an output port in optical communicationwith said optical receiver, wherein said optical switching deviceimparts a substantially uniform optical loss across the at least oneoptical signal channels; a distributor connected along the optical pathat an input port of the demultiplexer; at least one drop receiverconnected to an output port of the distributor; a combiner connectedalong the optical oath at an output port of the multiplexer; and atleast one add transmitter connect to an input port of the combiner. 2.The optical transmission system of claim 1, wherein the switch elementis configured to block wavelengths that are being dropped.
 3. Theoptical transmission system of claim 2, wherein the switch element isconfigured to block a signal group comprising a plurality ofwavelengths, wherein at least one of the wavelengths in the signal groupis being dropped.
 4. The optical transmission system of claim 2, whereinthe switch element is configured to block wavelengths that are beingadded.
 5. The optical transmission system of claim 1, wherein thedistributor is wavelength-selective and provides to the receiver onlysignals being dropped.
 6. The optical transmission system of claim 1,wherein the at least one add transmitter and the at least one dropreceiver are remotely located from the switching device.
 7. The opticaltransmission system of claim 1, wherein the at least one drop receiverand the at least one add transmitter are connected to a local opticaltransmission system.
 8. The optical transmission system of claim 7,wherein: at least one drop receiver is located in the local opticaltransmission system remote from the switching device; and at least oneadd transmitter is located in the local optical transmission systemremote from the switching device.
 9. The optical transmission system ofclaim 1, wherein: the at least one drop receiver is a tunable receiver;and the at least one add transmitter is a tunable transmitter.
 10. Theoptical transmission system of claim 1, wherein at least one of thedemultiplexer, mulliplexer and optical switching clement imparts anon-unifonn optical loss across the optical channel that is balanced byat least one of the demultiplexer, multiplexer and optical switchingelement to impart the substantially uniform optical loss across theoptical channels passing through the optical switching device.
 11. Theoptical transmission system of claim 10, wherein the optical switchingdevice includes at least one optical amplifier disposed between thedemultiplexer and the multiplexer to at least partially balance thenon-uniform optical loss imparted by at least one of the demultiplexer,multiplexer and optical switching element.
 12. An optical transmissionsystem comprising: at least one optical transmitter configured totransmit at least one optical signal channel via an optical signal alongan optical path; at least one optical receiver configured to receive atleast one optical signal channel from said optical path; and, an opticalswitching device disposed along said optical path including: ademultiplexer configured to demultiplex the optical signal provided toan input port into a plurality of signal groups, each signal groupincluding at least one signal channel, a switch element configurable topass or prevent the passage of at least one of the signal groups andvariably control the optical power of the signal groups passing throughsaid switch element, and a multiplexer configured to multiplex thesignal groups passed by said switch element into an output signalthrough an output port in optical communication with said opticalreceiver, wherein said optical switching device imparts a substantiallyuniform optical loss across the at least one optical signal channels; adistributor having an input port and a plurality of output ports andconfigured to distribute an optical add signal provided at the inputport into a plurality of optical signals at the output ports, wherein atleast one output port from the distributor is connected to at least oneinput port of the multiplexer via at least one switch element; acombiner having a plurality of input ports and an output port andconfigured to combine optical signals provided at the input ports intoat least one optical drop signal at the output port, wherein at leastone input port to the combiner is connected to at least one output portfrom the demultiplexer via at least one switch element.
 13. The opticaltransmission system of claim 12, wherein the distributor is ademultiplexer.
 14. The optical transmission system of claim 13, whereinthe input port of the distributor and the output port of the combinerare connected to a local optical transmission system.
 15. The opticaltransmission system of claim 14, wherein the distributor and thecombiner form a portion of an add/drop multiplexer in the local opticaltransmission system.
 16. The optical transmission system of clam 14,wherein the at least one output port of the distributor connected to theat least one input port of the combiner forms a through path in thelocal optical transmission system.
 17. The optical transmission systemof claim 14, further comprising a transmitter connected to the inputport of the distributor, wherein the transmitter is located in the localoptical transmission system and is remote from the switching device. 18.The optical transmission system of claim 17, further comprising areceiver connected to the output port of the combiner, wherein thereceiver is located in the local optical transmission system and isremote from the switching device.
 19. The optical transmission system ofclaim 12, wherein at least one output port of the distributor isconnected to at least one input port of the combiner.
 20. An opticalswitching device comprising: a demultiplexer including an input port anda plurality of output ports, wherein the demultiplexer is configured todemultiplex an, optical signal provided to the input port into aplurality of signal groups at the output ports, each signal groupincluding at least one signal channel, a switch element configurable topass or prevent the passage of at least one of the signal groups andvariably control optical power of the signal groups passing through saidswitch element, and a multiplexer configured to multiplex the signalgroups passed by said switch element into an output signal through anoutput port, wherein said optical switching device imparts asubstantially uniform optical loss across the at least one opticalsignal channels; a distributor connected at the input port of thedemultiplexer; at least one drop receiver connected to an output port ofthe distributor; a combiner connected at the output port of themultiplexer; and at least one add transmitter connect to an input portof the combiner.